Things may be better than ever for one pair of Castle crime-solvers, but for NYPD detectives Kevin Ryan and Javier Esposito, the tension can be cut with a knife.

As Season 5 of the ABC drama arrives this Monday at 10/9c, picking up just hours from where the finale ended, “There is a schism in the Force at the 12th precinct,” Seamus Dever, who plays Ryan, shared with TVLine during our visit to the set.

The issue at hand, of course, is Ryan’s decision in the aforementioned season-ender to loop in Captain Gates that Beckett and the boys were still investigating the murder of Kate’s mother. “Ryan did the right thing — and the end justifies the means in his mind,” says Dever. “Beckett is alive because of what he did,” seeing as it led to Kevin being able to pull her from the ledge where Cole Maddox left his prey hanging.

Esposito, however, didn’t and still doesn’t see it that way. As Jon Huertas notes, “Out of the ‘core four,’ Beckett and Esposito have known each other the longest, and Esposito is a pretty loyal guy, so for Ryan to go behind our backs and show a sort of disloyalty to our team really affects Esposito in a way that undermines his trust of Ryan.”

And that dissolution of trust, the compromise of their bromantic bond, will be evident in not just cold shoulders but “out-and-out hostility,” Dever previews. “Esposito is sort of fiery anyway, so yeah, it’s not going to be just chilly looks at each other.”

Luckily, as it were, Esposito was suspended by Gates for his off-book complicity, so he and Ryan won’t be crossing paths at the precinct straight away. Yet a hot lead on stone-cold Cole Maddox will reunite the partners for a common cause.

“We’re at Beckett’s trying to figure something out when Ryan just kind of shows up,” Huertas shares. “It’s the first time I’ve seen Ryan since I walked away from him in the precinct [the day before], and I don’t want to have anything to do with him — though Beckett tries to convince me otherwise.”

When I spoke to Dever and Huertas, it was apparent that “Rysposito” won’t be making nice any time soon. But when that time does come, what will it take to restore that trust and friendship?

“I broke a code in his mind — in a lot of people’s minds,” Dever says in character, “so there’s some mending that needs to happen.” Adds Huertas: “Ryan’s going to have to really work hard and do something spectacular to maybe change the tide. But it doesn’t happen in the first episode, and I don’t think it happens in the second, either.”

Which is kind of a shame, if only because the boys surely would love to compare notes as they each pick up on tiny clues that something is “different” between Beckett and Castle, who (“finally,” cheers Dever) hooked up in the Season 4 finale. To that end, though it’s Ryan who finds Rick in Kate’s apartment — fully clothed and in broad daylight, mind you — in one of the released promos, “There is something very telling that Esposito picks up on [in the premiere],” Huertas teases. “The great thing about him is he’s always been in tune with that vibe, that energy, so I think Esposito will probably be the first one to find out what’s going on.”Follow @MattMitovich